The present invention relates to a method of controlling insect populations, especially those which are herbivorous in nature, and in particular, to a method for effecting such control through biological means.
Over the years the existence of different species of insects which affect their local environment during one or more stages of their life cycles have at times had a harmful or deleterious influence on the local vegetation and/or indigenous creatures. Such environmental impact has many times proven harmful to the physical, economic, and social well being of humans. In recent times, examples of undesirable insects include the Gypsy Moth.sup.1, which essentially denudes broadleaf trees, and the Mediterranean Fruit Fly, which infests and destroys crops of fruit intended for human consumption. FNT .sup.1 See Marshall, E., Science, Vol. 213, dated Aug. 28, 1981, pp. 991-993.
Whether insects are directly detrimental to humans, such as those mentioned above, or merely locally upset the balance of nature, a definite need exists for control of insect populations or insects. Early control methods were primarily chemical in nature. More recent controls have been chemical and biological. Various methods were adopted for use against insects during vulnerable phases of insect life cycle, including toxic chemicals, introduction of sterile specimens into insect populations, the use of lures such as pheromones to attract insects, etc. Disease causing organisms, e.g., Bacillus thuringiensis, have also been used with some measure of success.
Which of the foregoing methods of control may be used will vary depending on the type of insect involved as well as the environment in which they are to be used. Unfortunately, the use of chemical agents for insect control has certain drawbacks such as unintended residual effect, indiscriminate lethality to living organisms, unpredictable long term effects on recipient organisms, persistence in food chains, and public disapproval of the use of chemical agents.
Consequently, it has become a desirable alternative to effect control of insect population through biological means. One method for such control is by seeding an insect population of a particular species with sterile male insects so that mating instincts are satisfied without the production of offspring. This method also has certain drawbacks in that it lacks immediacy and that it may not be sufficiently extensive in the face of a huge insect population.
Many of the problems associated with control of insect populations, such as selectivity, existence of residuum, lack of immediacy, etc., appear to be solved by the present invention which, in sum, induces the death of insects by a mechanism or mechanisms associated with wilt disease.
Wilt disease or "flackerie" in an insect is characterized by cessation of eating and sluggishness in activity, followed by complete motionlessness. After a period of time, usually several hours, dark putrified liquid oozes from body orifices such as the mouth and anus. The insect, e.g., caterpillar, becomes extremely flaccid, the legs losing their grip until finally, with only one or two of its false feet or with its anal claspers it hangs limp and dead. Its skin, which is by the time of death usually black, may be easily ruptured to emit a dark, thin liquid having an extremely offensive odor.
While wilt disease is best known as affecting the gypsy moth larva, Lymantria dispar Linnaeus, at least 35 other species of insects have been reported to be affected by the same or very similar diseases. See Stienhaus, Insect Microbiology: An Account of the Microbes Associated with Insects & Ticks, page 414 (1946). At first it was believed that a small, motile coccus, Gyrococcus flaccidifex, was the etiologic agent of the disease in the gypsy moth, however, extensive study in the area of gypsy moth wilt disease performed by R. W. Glaser and J. W. Chapman early in this century, led to a determination that wilt disease in the gypsy moth is caused by a filterable virus. The efficacy of this work is evident from the fact that the term "wilt disease" came to be used as a general term to designate any virus-caused disease of insects, id. at 421.
Further study regarding wilt disease led to the discovery of another micrococcus, i.e., Micrococcus flaccidifex danai, Index No. 3322-22120-2131, which has been identified as a causative agent of "wilt" disease in the Monarch Butterfly caterpillar, Danais plexippus; see American Museum Novitiates, No. 251, pages 5 and 6 (1927).
While these and possibly other etiological investigations into insect death by wilt disease have revealed different, lethal microorganisms, it is not believed that any wilt disease-inducing microorganism has been effectively used as a biological control of insect population.